I put caliber branded traction mats up the middle and ski guides front to back, including the ramp door, in my 2017 6x12 enclosed. Its never indoors, 24/7/365 outside. ZERO wooden floor issues. I just smash any ice chunks with a hammer and kick them out the back. Any water that puddles and freezes, if Im feeling lazy, ill throw a handful of sand on it.
Ive seen people use old truck bed liners as well. Kijiji will have some for sure.
I prefer to have only the mat and guides, not the whole floor covered. In the spring it'll air out better and I can broom or use a leaf blower to get standing water, leaves and dirt out.
A completely covered floor will retain moisture.